1. Field of the Invention
The present invention relates to a method of manufacturing a green light emitting diode (to be referred to as an LED hereinafter).
2. Description of the Related Art
A green LED is manufactured by epitaxially forming a p-n junction layer on a GaP crystal substrate by LPE (Liquid Phase Epitaxy). FIG. 3 is a sectional view of a main part of a green LED. Referring to FIG. 3, reference numeral 1 denotes an n-type semiconductor substrate doped with sulfur (S), tellurium (Te), or the like. N- and p-type layers 2 and 3 respectively containing, e.g., Si and Zn as a donor and an acceptor are sequentially formed on an N-type region of (111) major surface of the substrate 1. Subsequently, in order to obtain high luminous efficacy, nitrogen is doped in the N-type layer 2 to form a nitrogen (N) isoelectronic trap necessary for recombination light emission of excitons. In addition, an anode 4 is formed on the p-type layer 3, and a cathode 5 is formed on the lower surface of the semiconductor substrate 1. When a current flows between the anode 4 and the cathode 5 of the green LED having this arrangement, light emission occurs upon recombination of excitons trapped in the N isoelectronic trap as a luminescent center.
In this case, the emission wavelength is about 565 .mu.m, and the thicknesses of the n- and p-type layers 2 and 3 are set to be 25 .mu.m to 50 .mu.m.
The luminous efficacy is influenced by a concentration of nitrogen to be doped, a carrier concentration distribution near a p-n junction, and the lifetime of minority carriers injected in a junction. It is known that there is a high correlation between the lifetime of minority carriers and the etch pit density (to be referred to as the EPD hereinafter) of an epitaxial layer. The lifetime is abruptly shortened at an EPD of 1.times.10.sup.5 /cm.sup.2 or more, and the luminous efficacy is also decreased accordingly.
Since a GaP crystal has a wide band gap, an impurity level serving as a non-emission recombination center is easily set. In addition, in a green LED, since the light emission level is shallow, excitons trapped in the N isoelectronic trap are thermally dissociated, and the possibility that non-emission recombination occurs after the dissociation of excitons is high. For this reason, it is important to suppress the EPD of an epitaxial layer as a non-emission recombination center to 1.times.10.sup.5 /cm.sup.2.
Since the EPD of an epitaxial layer tends to copy the EPD of a substrate, it is important to use a low-EPD substrate for a green LED. However, a GaP single crystal used as a substrate is manufactured by LEC (Liquid Encapsulated Czochoralski) under a severe thermal environment of high temperature and pressure (1,467.degree. C. or more and 50 atm). For this reason, it is difficult to manufacture a single crystal having an EPD of 1.times.10.sup.5 /cm.sup.2 with a high yield, and demands for stable supply and a decrease in cost cannot be satisfied.
As described above, in order to increase the luminous efficacy of a GaP green LED, it is very important to suppress the EPD of an epitaxial layer contained in the GaP substrate to a low value. In conventional techniques, however, since GaP crystals are grown under a severe environment of high temperature and pressure, it is difficult to stably manufacture an ingot having a sufficiently low EPD at a low cost. A strong demand has arisen on the market for a wider application range of green LEDs and higher external luminous efficacy.